Time and Tense Markers
Thai verbs never change. Time comes from small marker words and from ordinary time expressions.
Time is a word, not an ending
There is no past tense in Thai, in the sense of a changed verb. pai is go, went, will go, has gone. What decides which is either a time word in the sentence or the fact that everyone already knows. Much of the time you need nothing at all. If you say 'yesterday I go market', the yesterday has done the work and no further marking is wanted. Over-marking is a much more common learner error than under-marking. The three markers worth knowing are ja (จะ) for the future, laeo (แล้ว) for something completed, and kamlang (กำลัง) for something in progress.
The three markers
ja (จะ) sits in front of the verb and points at the future: ja pai, will go. It carries intention as much as time, so it also covers 'I am going to'. laeo (แล้ว) sits at the end and says the thing is done or the state has changed: kin laeo, have eaten; im laeo, full now. kamlang (กำลัง) sits in front of the verb for an action in progress, and is often paired with yu (อยู่) at the end: kamlang kin yu, in the middle of eating. Thai uses this far less than English uses its -ing, because context usually makes it obvious.
The past, when you must mark it
dai (ได้) in front of a verb marks a past event, usually one being confirmed or contrasted: phom dai pai laeo, I did go. It is not needed for ordinary past statements and sounds heavy if you sprinkle it everywhere. khoei (เคย) in front of a verb means 'have ever', the experience past: khoei pai chiang mai mai, have you ever been to Chiang Mai? The answer is khoei or mai khoei. The combination yang mai khoei (ยังไม่เคย) is 'never yet', which is the friendliest way to admit you have not done something.
Time words that carry the sentence
In practice a handful of ordinary words do most of the tense work: muea wan (เมื่อวาน) yesterday, wan nii (วันนี้) today, phrung nii (พรุ่งนี้) tomorrow, ton nii (ตอนนี้) now, muea kii (เมื่อกี้) just now, thi lang (ทีหลัง) later. They go at the front of the sentence more often than the end, and once one is present you can drop the markers entirely. For how often, thuk (ทุก) means every: thuk wan every day, thuk khon everyone. bang khrang (บางครั้ง) is sometimes.
Talking about when
Read these and notice how few markers there are. Most of the timing lives in the ordinary words, exactly as it does for a Thai speaker.
Marking every sentence
The strongest habit English brings is the need to mark tense on every verb. In Thai that produces sentences stuffed with ja and laeo where a Thai speaker would use neither. Rule of thumb: if a time word is already in the sentence, or if the timing is obvious from what you were just talking about, use no marker at all.
Reading laeo as past tense
laeo is not a past marker. It marks completion or a change of state, which often coincides with the past but not always. im laeo means 'I am full now', a present state that has just changed. pai laeo can mean 'he has gone' or 'off we go'. Think 'already' or 'now' rather than 'ed'.
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